In a variable air volume (VAV) temperature conditioning system, the volume of conditioned air supplied to each comfort zone in a building is proportional to the demand in that zone. Stages of temperature conditioning are energized as required to maintain the supply air at a relatively constant temperature as the total demand on the system varies.
A VAV system often includes an outdoor air economizer for admitting fresh air into the building, thereby reducing the need for mechanical cooling. Depending upon the enthalpy of the outdoor air, a damper assembly on the economizer may be modulated between minimum and maximum positions to control the proportion of outdoor air used for cooling the building interior spaces. If an enthalpy sensor indicates that outdoor air temperature/humidity is excessive, the economizer damper is kept at its minimum position.
Staging controls for VAV systems typically attempt to satisfy the cooling demand first with the outdoor air economizer, and if that is inadequate, thereafter by energizing successive cooling stages. Of course, if the outdoor ambient air temperature is less than the setpoint temperature of the discharge air, outdoor air can be used to cool the building without energizing any refrigerant cooling stages. Conversely, if the outdoor ambient air temperature is greater than the setpoint temperature, it is unlikely that the cooling demand can be met entirely with air supplied by the economizer.
After the first cooling stage is energized, such controls generally either modulate the economizer damper to its minimum open position, or leave it at maximum opening. However, if the economizer is left at maximum open condition when one or more stages of cooling are energized, it is very possible that the system control will energize hot gas bypass, in response to a low compressor suction pressure. The term "hot gas bypass" refers to an operating mode wherein vapor is supplied directly to the evaporator, bypassing the condenser. This mode is implemented by the compressor protection circuit if required to vaporize excess liquid refrigerant in the evaporator, to prevent it from "slugging" the compressor. Low suction pressure may occur when a large volume of relatively cold outdoor air entering through a fully open economizer mixes with and lowers the return air temperature in the VAV duct below that required to vaporize all the refrigerant in the evaporator.
Although use of hot gas bypass prevents damage to a compressor under the above-noted conditions, it is undesirable to operate a cooling system in this manner. Assuming the outdoor ambient air temperature is greater than the setpoint temperature and one or more stages of cooling are energized, it is much more efficient and economical to reduce the use of outdoor air admitted to the economizer to a level at which the evaporator is adequately loaded, while maintaining discharge air temperature at the setpoint. It is likewise generally inefficient to close the economizer to its minimum open position as soon as the stage of cooling is energized, even though this avoids use of hot gas bypass. At minimum economizer opening, the system may not be taking advantage of the cooling available in outdoor air. Even if the temperature of the outdoor air is greater than the setpoint, it is often less than the return air temperature, making it beneficial to use more than a minimum of outdoor air to cool the building.
In consideration of the above, it is an object of the subject invention to control a plurality of refrigerant compressor cooling stages and an outdoor air economizer in a manner which achieves efficient, low cost operation of a cooling system.
Another object of this invention is to modulate the outdoor air economizer of a VAV cooling system in a manner which both minimizes the use of mechanical refrigerant cooling stages, and maintains a relatively constant discharge air temperature.
Yet another object of this invention is to modulate the outdoor air economizer to limit the proportion of outdoor ambient air which is mixed with the return air sufficiently to minimize use of hot gas bypass, if the outdoor air temperature is warmer than the discharge air temperature and one or more stages of cooling are energized.
A further object of this invention is to control stages of refrigerant cooling to minimize cycling, thereby extending the expected operating life of the refrigerant compressor.
A still further object of this invention is to provide a selectable automatic reset of the discharge air temperature setpoint as a function of the outdoor air temperature.
These and other objects of the subject invention will become apparent from the description of the preferred embodiment which follows, and by reference to the attached drawings.